Off-the-road tire

ABSTRACT

The invention is to improve wear resistance and resistance to crack growth of an off-the-road tire having a gauge of a tread rubber of 60-200 mm, in which a plurality of widthwise grooves extending substantially in a widthwise direction of the tire are arranged in each side region of the tread, and a pair of circumferential fine grooves continuously extending straight-forward or zigzag in a circumferential direction of the tire are arranged in a central zone of the tread, and a plurality of widthwise fine grooves extending substantially in the widthwise direction of the tire and contacting their grooves walls with each other in a ground contact area are arranged between these circumferential fine grooves.

TECHNICAL FIELD

[0001] This invention relates to an off-the-road tire capable ofsimultaneously establishing the wear resistance and resistance to crackgrowth of a tread at a higher level even under severer load conditionand traction condition.

BACKGROUND ART

[0002] Since the off-the-road tires used for construction vehicle andthe like are required to have considerably high load capacity andtraction performance as compared with tires for truck and bus, it isgeneral to have a tread pattern that ribs capable of developing a highwear resistance under a high load are formed on a central portion of thetread and lugs capable of developing a high traction performance areformed on both side portions of the tread.

[0003] However, when a width of a zone forming straight or zigzag ribsin the central portion of the tread sandwiching between phantom lineseach connecting tops of lug grooves to each other is made wider thannecessary, traction performance during the running on an uphill grade orthe like is lacking, and also there is a drawback that lateral slippageis apt to be easily caused in the steering because circumferentialgroove(s) is not naturally existent in the central portion. For thisend, so-called lug-block mix pattern in which block rows producing anedge effect in a widthwise direction of the tread are formed in thecentral portion of the tread are used at the present time.

[0004] On the other hand, the tire is recently and strongly demanded tohave further improvements of the load capacity and traction performanceand an improvement of wear resistance with the further advance ofvehicle performances. In order to satisfy such a demand, there is atendency that the tire size is made large and the thickness of the treadis made thicker. In this case, it is particularly feared that heatgeneration in the central portion of the tread exerts upon thedurability of the tire.

[0005] For the purpose of obtaining a tread pattern capable of beingdurable to excessive traction and realizing an excellent resistance tolateral slipping and enhancing heat radiation while leaving continuousrib rows are left in the central portion of the tread as far as possibleconsidering the wear resistance, therefore, it is attempted to arrange apair of circumferential fine grooves for heat radiation in the centralportion of the tread at a required minimum groove width.

[0006] In the off-the-road tire provided with the circumferential finegrooves for heat radiation, however, when the block rows are formed inthe central portion of the tread as mentioned above, each block easilymoves in front and behind or in left and right during the running of thetire under loading, so that the demand for high load capacity and hightraction performance can not be satisfied and there are caused problemsrelating to the wear resistance such as premature wearing of tread,block chipping and other troubles.

[0007] On the other hand, when rows of land portion continuing more thannecessary in the circumferential direction are formed on the centralportion of the tread in such an off-the-road tire, if cut failureproduced on the surface of the land portion row arrives at the beltlayer, a large stress is produced resulting from the fact that the thickland portion rows are continuously arranged in the circumferentialdirection whenever the tread is largely deformed in the circumferentialdirection by subsequently large traction force, whereby there is causeda problem relating to the resistance to crack growth that the cutfailure progresses as a crack between the tread and the belt in thecircumferential direction of the tire along a propagation direction ofsuch stress and further so-called cut separation of completelyseparating the tread from the belt is easily developed.

[0008] The invention is to solve the above problems inherent to theconventional technique and to provide an off-the-road tire capable ofproviding a high wear resistance without causing troubles such aspremature wearing of the tread, block chipping and the like even underthe exposure to severer load condition and traction condition, andcapable of providing an excellent resistance to crack growth that evenif cur failure is produced on a ground contact face of the tread, ithardly progresses into a crack between the tread and the belt layer.

DISCLOSURE OF THE INVENTION

[0009] The off-the-road tire according to the invention is characterizedin that a gauge of a tread rubber is within a range of 60-200 mm, and aplurality of widthwise grooves extending substantially in a widthwisedirection of the tire are arranged in each side region of the tread, anda pair of circumferential fine grooves continuously extendingstraightforward or zigzag in a circumferential direction of the tire arearranged in a central zone of the tread, and a plurality of widthwisefine grooves extending substantially in the widthwise direction of thetire and contacting their grooves walls with each other in a groundcontact area are arranged between these circumferential fine grooves.

[0010] Moreover, the central zone of the tread is a zone correspondingto not more than 50% of a tread width centering on an equatorial line ofthe tire.

[0011] In this off-the-road tire, a high heat radiation of the tread inaddition to the resistance to lateral slipping can be obtained by thecircumferential fine grooves. On the other hand, the widthwise finegrooves in the central zone of the tread are closed at a state ofcontacting their groove walls with each other during the running of thetire under loading and act as such a land portion that the adjoiningblocks in the circumferential direction of the tire are substantiallycontinued in the circumferential direction, so that the blocks in theblock row located in the central zone of the tread hardly deform infront and behind or in left and right even in the running of the tireunder loading. As a result, the occurrence of troubles such as prematurewearing of the tread, block chipping and the like can be effectivelyprevented to develop an excellent wear resistance even under extremelyhigh load condition and excessive traction condition.

[0012] In such an off-the-road tire, the plural widthwise fine groovesare arranged in the central zone of the tread to prevent the landportion row in the central zone of the tread from the continuing morethan necessary. Therefore, even if cut failure generated in the landportion row, i.e. the blocks of the block row arrives at the belt layer,the continuity of the land portion is prevented by the widthwise finegrooves to reduce stress of strain deformation in the fine grooveportion as compared with that in other portions with respect to a largestrain deformation of the tread in the circumferential direction of thetire, and hence the growth of the cut failure as a crack between thetread and the belt layer in the circumferential direction of the tirecan be prevented by the widthwise fine grooves, so that an excellentresistance to crack growth can also be developed effectively.

[0013] According to the off-the-road tire, therefore, the excellent wearresistance can be realized based on the selected width of the widthwisefine groove without bringing about the troubles such as prematurewearing of the tread, block chipping and the like even if the action ofextremely high load and excessive traction is applied to the tire, whilethe excellent resistance to crack growth which hardly progresses the cutfailure produced in the land portion row of the central zone into thecrack between the tread and the belt layer can be realized based on thepresence of the widthwise fine grooves itself.

[0014] Preferably, the off-the-road tire is produced by vulcanizing aregion from a bead portion of a product tire to a sidewall portion and atread shoulder portion thereof through a mold part of a full mold formand a region corresponding to a central portion of the tread locatedfrom the tread shoulder portion toward a tread center through a moldpart of a split mold form, respectively.

[0015] As the off-the-road tire, there are frequently super-large sizetires of a lug-block mix pattern structure having a thicker tread andrelatively deep circumferential fine grooves in its central portion. Incase of vulcanization building such an off-the-road tire, it is possibleto conduct vulcanization through only so-called split mold. In thiscase, since the tire size is vary large, each segment constituting thesplit mold becomes considerably expensive and the manufacture thereof isrequired to take a greater number of steps and also it is difficult torapidly cope with the change of tire size, alteration of pattern and thelike requiring the change of each segment.

[0016] On the other hand, when the off-the-road tire is built byvulcanization through only so-called full mold, the exchange, alterationof the pair of the mold parts and the like can be conducted relativelyeasily, but there is a fear that when the tire is removed off from themold, the tread is damaged in the width-narrow circumferential finegroove portions having a significant groove depth or the like. 02831(PC[/JPO2/05670)

[0017] In the invention, therefore, the vulcanization is carried out byusing the mold part of full mold form comprised of two upper and lowermold segments conducting the mold opening substantially in an axialdirection of the tire with respect to the region from the bead portionof the product tire to the sidewall portion and tread shoulder portionthereof, and by using the mold part of spilt mold form comprised ofplural divided segments in the circumferential direction of the tireconducting the mold opening substantially in the radial direction of thetire with respect to the region corresponding to the central portion ofthe tread located from the tread shoulder portion toward the treadcenter, whereby the number of mold manufacturing steps, the mold costand the like can be reduced but also the change of tire size, alterationof pattern and the like can be conducted quickly and further the fear ofdamaging the circumferential groove portions can be sufficientlyremoved.

[0018] In the alteration of the tread pattern of the tire to bevulcanized, it is possible to alter only the mold part of the split moldform while the mold part of the full mold form having a high versatilityis used as it is, or in other words, the widthwise grooves formed in thetread shoulder zone are common. Thus, a time of exchanging the mold as atime for waiting the vulcanization can be advantageously shortened andalso the total cost of the mold can be reduced.

[0019] In the invention, a boundary between the mold part of the fullmold form and the mold part of the split mold form is made incorrespondence with a groove edge of a straight or zigzagcircumferential fine groove to be formed in the tread and located towarda side edge of the tread, and the vulcanization is conducted in a moldof these mold parts located along the groove edge to produce anoff-the-road tire. In the mold opening of these mold parts after thevulcanization, a fear of damaging the tread in the vicinity of thegroove portion is removed, and also spew rubber overflowed between thesemold parts, which damages the uniformity of ground contact pressure ofthe land portion to easily generate a nucleus of uneven wear and easilyrendered into a nucleus for tread crack as it is, can be positioned onthe groove edge, whereby an influence of the spew rubber upon theoccurrence of uneven wear and crack can effectively be controlled.

[0020] When the circumferential fine groove is zigzag, the groove volumeper unit circumferential length of the tire can be made larger than thatof the straight groove to further enhance heat radiation effect throughthe circumferential fine groove.

[0021] Further, when the off-the-road tire is produced by vulcanizingthrough a mold in which the boundary between the mold parts as mentionedabove is straightforward positioned in correspondence with a groove edgeof a zigzag circumferential fine groove to be formed in the tread andlocated nearest to a side edge of the tread, the damage of the tread inthe mold opening after the vulcanization can be prevented but also thespew rubber between these mold parts can be positioned in the landportion of the tread side zone producing a relatively low ground contactpressure, whereby the spew rubber resulting in the occurrence of unevenwear and forming the nucleus for crack can be advantageously blocked.

[0022] Moreover, when the tire is produced by vulcanizing through a moldin which the boundary between the mold parts is positioned incorrespondence with the land portion defined by the circumferential finegroove to be formed in the tread toward the side edge of the tread, thespew rubber between these mold parts is positioned in place having alower ground contact pressure, whereby the fear of resulting in theuneven wear and the like can be sufficiently removed.

[0023] On the other hand, the width of the mold part of the split moldform becomes relatively wider, so that the degree of freedom in thealteration of the tread pattern can be enhanced.

[0024] In such an off-the-road tire, when a groove width of thewidthwise groove is 25-80 mm, the traction performance based on theaction of the tread side zone and the wear resistance of such a sidezone can be well established, while when a groove depth of the widthwisegroove is 50-160 mm, the wear resistance of the tread side zone and theresistance to heat generation can be well established.

[0025] The term “groove width” used herein means an average value ofwidths measured in an existing direction of a groove center line over50% of a central region of the center line, while the term “groovedepth” means an average value of depths measured in an existingdirection of a groove center line over 50% of a central region of thecenter line.

[0026] When the groove width of the circumferential fine groove is15-50% of the groove width of the widthwise groove, the necessary andsufficient heat radiation effect is ensured but also the rigidity of thecircumferential fine groove forming portion, which corresponds toapproximately a half of a tread half-width and is apt to be relativelyworn, can be enhanced by contacting groove walls with each other in theground contact face, so that the rigidity of the land portion as a wholeof the tread can be increased to improve the wear resistance.

[0027] Also, when the groove depth of the circumferential fine groove is60-100% of the groove depth of the widthwise groove, the contacting ofthe groove walls in the ground contact face can easily be realized.

[0028] Preferably, the groove width of the widthwise fine groove is15-50% of the groove width of the widthwise groove. In this case, thegrowth of cracks between the tread and the belt resulted from the cutfailure is effectively prevented by the widthwise fine grooves, while anexcellent wear resistance can be given to the land portion of thecentral zone of the tread by contacting groove walls with each other.

[0029] In other words, when the groove width is less than 15%, thecircumferential length of a thin portion beneath the groove as a regionof preventing the crack growth is too short and there is a fear that thewidthwise fine groove can not sufficiently develop an expected function,and also a width of a projection in a mold for the formation of thegroove, i.e. a thickness thereof is too thin and the lowering of thedurability of the projection can not be avoided. While, when it exceeds50%, it is difficult to contact the groove walls in the ground contactface with each other or contact the adjoining land portions in thecircumferential direction with each other and there is left a fear ofchipping the land portions or the like.

[0030] Preferably, the groove depth of the widthwise fine groove is60-100% of the groove depth of the widthwise groove. In this case,excellent resistance to crack growth and protecting property to tirecasings such as belt, carcass and the like can be simultaneouslyrealized.

[0031] When the groove depth is less than 60%, the rubber gauge of thetread beneath the widthwise fine groove is too thick, and stress in thecentral zone of the tread during the running of the tire, which resultsin the growth of the cut failure arrived at the belt layer in thecircumferential direction of the tire, can not be sufficiently reduced,so that it is difficult to realize the excellent resistance to crackgrowth. While, when it exceeds 100%, the rubber gauge beneath thewidthwise fine groove is too thin and the tire casings such as belt,carcass and the like can not sufficiently be protected.

[0032] In such an off-the-road tire, an average intersecting angle ofthe widthwise fine groove at an acute angle side with respect to theequatorial line of the tire is preferably 45-90°.

[0033] The term “average intersecting angle” used herein means an angleof a line segment connecting intersects between the circumferential finegroove and the widthwise fine groove to each other with respect to theequatorial line of the tire.

[0034] In this case, the excellent wear resistance can be realized. Thatis, when the intersecting angle is less than 45°, it is difficult tocontact the groove walls of the widthwise fine groove with each otherduring the running of the tire under loading and also it is difficult tocooperate the adjoining land portions in the circumferential directionas a land portion body continuing substantially in the circumferentialdirection and hence it is difficult to realize the excellent wearresistance. Further, an acute corner part is created in the land portiondefined by the widthwise fine grooves to easily chip such a corner partduring the running of the tire under loading and hence it is difficultto realize the excellent wear resistance.

[0035] Preferably, a JIS A hardness of tread rubber in the central zoneof the tread is 55-80, while a loss tangent tan δ (25° C.) thereof is0.05-0.35.

[0036] The term “JIS A hardness” used herein means a hardness measuredat a testing temperature of 25° C. using a type A testing machine for adurometer hardness test according to JIS K6253-1993, while the term“loss tangent tan δ (25° C.)” means a value that a loss tangent tan δrepresented by a ratio of loss modulus to dynamic modulus is measured ata testing temperature of 25° C. under conditions of initial static loadof 1.6 N, average strain amplitude of 1% and testing frequency of 52 Hzusing a spectrometer made by Toyo Seiki Co., Ltd. according to “casethrough load wave, deflection wave (tension)” of JIS K6394-1995.

[0037] In this off-the-road tire, a high rigidity of the land portion isensured and the excellent resistance to crack growth can be provided byrendering the hardness of the tread rubber at the central zone of thetread into 55-80, while the excellent heat resistance can be developedwhile ensuring the necessary rigidity of the land portion by renderingthe loss tangent tan δ (25° C.) into 0.05-0.35.

[0038] Preferably, the groove width of the circumferential fine grooveis a width contacting the groove walls with each other in the groundcontact face. In this case, the rigidity of portions forming thecircumferential fine grooves and hence the rigidity of the land portionsin the whole of the tread can be enhanced by contacting the groove wallsof the circumferential fine groove during the running of the tire underloading as previously mentioned to develop the excellent wearresistance.

[0039] Also, it is preferable that the block defined by thecircumferential fine grooves and the widthwise fine grooves as a landportion is triangular, quadrilateral or hexagonal. In this case, theblocks can be continuously arranged in the circumferential direction toattain a uniform block row in the circumferential direction whileensuring the block rigidity required in the central zone of the tread.In other words, a portion having a low rigidity may be produced if theshape of the block is complicated.

[0040] The off-the-road tire having the above construction is preferableto be produced by vulcanizing and building in a mold wherein the widthof the projection for the formation of the circumferential fine grooveis made smaller than the width of the projection for the formation ofthe widthwise fine groove.

[0041] In such an off-the-road tire, it is general to adopt such a beltstructure that the laminating number of belt layers is large in thecentral zone of the tread and small in the side zone thereof for wellestablishing the control of size growth in the central zone of the treadand the prevention on the occurrence of troubles at the side edge of thebelt and the neighborhood thereof. According to this belt structure, aside edge of a narrow-width belt layer contributing to the reinforcementof the central zone of the tread is frequently existent in a positioncorresponding to approximately 25% of a tread width measured from theequatorial plane of the tire or the neighborhood thereof.

[0042] For this end, the size growth in the vicinity of the side edge ofthe narrow-width belt layer particularly tends to be larger than that ofthe adjoining portion thereof in the inflation of an air pressure intothe tire and during the running of the tire under loading.

[0043] Therefore, the groove width of the circumferential fine grooveusually extending in the vicinity of the side edge of the narrow-widthbelt layer and tending to widen the groove width accompanied with thesize growth is made approximately equal to the groove width of thewidthwise fine groove extending in the central zone of the tread havinga small size growth in the use of the tire and tending to narrow thegroove width through the pushing of the surrounding tread rubber, whichis attained by selecting the respective widths of the projections forthe formation of the respective fine grooves, so that it is preferablethat as to the relative widths of the respective fine grooves at a timeof vulcanizing and building the tire, the circumferential fine groove ismade narrower than the widthwise fine groove considering the influenceof the size growth as mentioned above.

[0044] More preferably, the groove bottom of straightforward extendingportions in the zigzag extending circumferential fine groove is renderedinto a raised bottom at an end portion located toward a side edge of thetread. According to this structure, the increase of the groove widthresulted from the size growth of the tire can be effectively controlledbased on the increase of the groove bottom rigidity through the raisedbottom portion, and also the occurrence of cracks at the groove bottomresulted from the large enlargement of the groove width can beprevented.

[0045] On the other hand, when the groove bottom of the straightforwardextending portions in the zigzag extending circumferential fine grooveis rendered into a raised bottom at an end portion located toward theside of the tread center, the protrusion of the surface of the landportion at the side edge of the tread outward from the surface of theland portion at the side of the tread center in the radial direction dueto the difference of expansion amount in the radial direction betweenthe land portion located at the side of the tread center and the landportion located at the side of the side edge of the tread, which aredefined by the circumferential fine grooves and resulted from thedifference in the influence of the size growth thereupon, can becontrolled based on the rigidity of the raised bottom portion connectingboth the land portions and hence deformation restraining force.

[0046] When a surface height of a corner part in the land portionadjacent to the side edge of the tread defined by the zigzag extendingcircumferential fine grooves is gradually decreased toward a top sidethereof, even if the surface of the land portion located toward the sideedge of the tread is relatively protruded outward from the surface ofthe land portion located at the side of the tread center in the radialdirection under an influence of the size growth, the formation of thecorner part as a nucleus for uneven wear can be advantageously preventedby decreasing the surface height of the corner part toward the top sidethereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a developed view of a tread pattern showing anembodiment of the invention.

[0048]FIG. 2 is a diagrammatically section view of a main part in aradial direction illustrating a boundary position of a mold part.

[0049]FIG. 3 is a schematic view illustrating a boundary position of amold part in connection with a tread pattern.

[0050]FIG. 4 is a developed view of a tread pattern showing anotherembodiment of the invention.

[0051]FIG. 5 is a diagrammatic view illustrating the other embodiment.

[0052]FIG. 6 is a diagrammatic view illustrating a further embodiment.

[0053]FIG. 7 is a diagrammatic view illustrating a still furtherembodiment.

[0054]FIG. 8 is a developed view of a tread pattern in the conventionaltire.

[0055]FIG. 9 is a developed view of a tread pattern in anotherconventional tire.

BEST MODE FOR CARRYING OUT THE INVENTION]

[0056] In FIG. 1 is shown a developed view of a tread pattern, in whichnumeral 1 is a tread.

[0057] In this embodiment, the tread 1 has a rubber gauge of 60-200 mm,and a plurality of widthwise grooves 2 extending substantially in awidthwise direction of the tire are arranged in each side zone of thetread 1 and a pair of circumferential fine grooves 3 continuouslyextending in a circumferential direction of the tire are arranged in acentral zone of the tread 1, and a plurality of widthwise fine grooves 4extending substantially in the widthwise direction of the tire andopening to both circumferential fine grooves 3 are arranged between boththe circumferential fine grooves. In this case, the groove width of thewidthwise fine groove 4 is a width contacting groove walls with eachother in a ground contact face.

[0058] In the illustrating embodiment, the pair of the circumferentialfine grooves 3 are extended zigzag in the circumferential direction, butit is possible to extend them in a straight form.

[0059] The tire having such a tread pattern can be produced byvulcanizing a region corresponding to a bead portion and a sidewallportion and a side portion of a tread of a product tire with a mold part5 of a so-called full mold form and a region corresponding to a centralportion of the tread located from the side portion of the tread toward atread center with a mold part 6 of a so-called split mold form as shown,for example, by a radially section in FIG. 2. In this case, it ispreferable that a boundary C between the mold part 5 of the full moldform and the mold part 6 of the split mold form is positioned incorrespondence with a groove edge 3 a of the straight or zigzagcircumferential fine groove 3 formed in the tread 1 and located toward aside edge of the tread and along the groove edge 3 a.

[0060] Moreover, the boundary C between both the mold parts 5 and 6 maybe straightforward positioned in correspondence with a groove edge ofthe zigzag circumferential fine groove 3 formed in the tread 1 andlocated nearest to the side edge of the tread, or positioned incorrespondence with a land portion defined from the circumferential finegroove 3 formed in the tread 1 toward the side edge of the tread such asa lug 7.

[0061]FIG. 3 is a schematic view illustrating the position of the aboveboundary between the mold parts in connection with a tread pattern,wherein FIG. 3a shows a case that the boundary C is positioned incorrespondence with the groove edge 3 a of the zigzag circumferentialfine groove 3 located side the side edge of the tread along therewith,and FIG. 3b shows a case that the boundary C is positioned incorrespondence with the groove edge 3 a of the straight circumferentialfine groove 3 located side the side edge of the tread along therewith.

[0062]FIG. 3c shows a case that the boundary C is straightforwardpositioned in correspondence with the groove edge 3 a of the zigzagcircumferential fine groove 3 located nearest to the side edge of thetread along therewith, and FIG. 3d shows a case that the boundary C ispositioned in the lugs 7 defined from the circumferential fine groove 3toward the side edge of the tread.

[0063] In the thus produced tire, it is preferable that the groove widthof the widthwise groove 2 is 25-80 mm and the groove depth thereof is50-160 mm. Also, it is preferable that the groove width of thecircumferential fine groove 3 is 15-50% of the groove width of thewidthwise groove 2 and the groove depth thereof is 60-100% of the groovedepth of the widthwise groove 2.

[0064] Such numerical ranges for the circumferential fine groove 3 arealso applied to the widthwise fine groove 4. That is, it is preferablethat the groove width of the widthwise fine groove 4 is 15-50% of thegroove width of the widthwise groove 2 and the groove depth thereof is60-100% of the groove depth of the widthwise groove 2.

[0065] Also, it is preferable that an average intersecting angle θ ofthe widthwise fine groove 4 at its acute angle side with respect to anequatorial line E of the tire is 45-90°.

[0066] And also, it is preferable that JIS A hardness of a tread rubberin the central zone of the tread is 55-80 and a loss tangent tan δ (25°C.) is 0.05-0.35.

[0067] Furthermore, it is preferable that the groove width of thecircumferential fine groove 3 is rendered into a width of contactinggroove walls with each other in a ground contact face, and that blocks 8in the central zone of the tread defined by the circumferential finegrooves 3 and the widthwise fine grooves 4 are triangular, quadrilateralor hexagonal as illustrated.

[0068] According to the above off-the-road tire, the excellent wearresistance can be developed without causing troubles such as prematurewearing, block chipping and the like even at a state of applying anextremely high load and an excessive traction but also the excellentresistance to crack growth hardly growing from cut failure produced onthe surface of the block to crack between the tread and the belt layer,and further fears such as thermal fatigue and the like due to heatgeneration of the tread 1 can be sufficiently eliminated by heatradiation of the circumferential fine grooves 3.

[0069]FIG. 4 is a developed view of a tread pattern showing anotherembodiment, in which numeral 12 is a widthwise groove, numeral 13 acircumferential fine groove and numeral 14 a widthwise fine groove.

[0070] This is the same construction as in FIG. 1 except that the groovewidth of the circumferential fine groove 13 is made wider to a levelthat the groove walls are not contacted in the ground contact face andthe groove depth thereof is made shallower than that of FIG. 1 and thegroove depth of the widthwise fine groove 14 is made shallower than thatof FIG. 1.

[0071] Even in this tire, the excellent wear resistance can be developedeven at a state of applying an extremely high load and an excessivetraction likewise the above case, and also excellent resistance to crackgrowth and heat radiation function of the tread can be developed.

[0072] In the off-the-road tire as shown in FIG. 1, when the groovewidth of the circumferential fine groove 3 is made approximately equalto, for example, that of the widthwise fine groove 4 at a state of usingthe tire filled with an air pressure, it is preferable that a width sizeof a projection for the formation of the circumferential fine groove ina vulcanization mold is made smaller than a width size of a projectionfor the formation of the widthwise fine groove considering the amount oftire size growth as previously mentioned. Thus, the groove widths of thecircumferential fine groove and the widthwise fine groove can be easilymade approximately equal based on the fact that the amount of sizegrowth accompanied with the filling of the cir pressure into a producttire is larger in a portion forming the circumferential fine groove thanin a portion forming the widthwise fine groove.

[0073] In order to suppress the enlargement of the groove width of thecircumferential fine groove due to the above size growth of theoff-the-road tire, as shown in FIG. 5, it is preferable that a raisedbottom portion 23 c as shown at a section in FIG. 5b is arranged on anend part of a groove bottom 23 b of a straightforward extending portion23 a in a zigzag extending circumferential fine groove 23 located sidethe side edge of the tread and the rigidity of the groove bottom isenhanced by such a raised bottom portion 23 c.

[0074] In case of considering that the size growth of the off-the-roadtire becomes particularly large in the vicinity of the side edge of thenarrow-width belt layer reinforcing the central zone of the tread orfrequently in a portion of the circumferential fine groove adjacent tothe side edge of the tread, as shown in FIG. 6, it is preferable that araised bottom portion 23 d as shown at a section in FIG. 6b is arrangedon an end part of a groove bottom 23 b of a straightforward extendingportion 23 a in a zigzag extending circumferential fine groove 23located toward the t read center, whereby the protrusion of the surfaceof the lug 27 outward from the surface of the block 28 in the radialdirection is suppressed by the rigidity of the raised bottom portion 23d connecting both to each other. Thus, the protruding amount of the lugsurface to the block surface can be suppressed to a small level.

[0075] In stead of the above or in addition thereto, as shown in FIG. 7,when a slant face 27 a gradually decreasing a surface height thereoftoward a top side is arranged on a corner part defined by thecircumferential fine groove 23 and adjoining to the side edge of thetread or a corner part of the lug 27 in the illustrated embodiment, theprotrusion of the surface of the corner part outward from the blocksurface is effectively prevented by the action of the slant face 27 aand also the rigidity of the corner part itself can be enhanced andhence the formation of the corner part as a nucleus for uneven wear andthe premature progress of the uneven wear can be preventedadvantageously.

EXAMPLE 1

[0076] There are prepared off-the-road tires according to the inventionand performance evaluations with respect to wear resistance, resistanceto crack growth, heat radiation and traction performances are carriedout as mentioned below.

[0077] Example tire 1 has a tread pattern of FIG. 1, and Example tire 2has a tread pattern of FIG. 4, and Conventional tire 1 has a treadpattern of FIG. 8, and Conventional tire 2 has a tread pattern of FIG.9, respectively. Also, Example tire 3 has a groove structure of FIG. 5,and Example tire 4 has a groove structure of FIGS. 5 and 6, and Exampletire 5 has a groove structure of FIGS. 5, 6 and 7.

[0078] Further, each of these test tires has dimensions as shown inTable 1, in which a region ranging from a bead portion to a sidewallportion has a structure similar to that of an off-the-road tire for ageneral construction vehicle.

[0079] Moreover, each test tire has a tire size of 4000R57, and a rimhaving a width of 29 inches and a flange height of 6 inches is used, andan air pressure of the tire is 686 kPa, and a load is 588 kN. TABLE 1Example Example Conventional Conventional Example Example Example Unittire 1 tire 2 tire 1 tire 2 tire 3 tire 4 tire 5 Thickness of tread atequatorial plane of tire mm 110 110 110 110 110 110 110 Thickness oftread at ground contact end mm 140 140 140 140 140 140 140 JIS Ahardness at central zone of tread ° 63 63 63 63 63 63 63 Tan δ (25° C.)at central zone of tread — 0.16 0.16 0.16 0.16 0.16 0.16 0.16 Averagegroove width of widthwise groove mm 70 70 70 70 70 70 70 Average groovedepth of widthwise groove mm 95 95 95 95 95 95 95 Average groove widthof circumferential fine groove/ — 0.25 0.45 0.45 — 0.25 0.25 0.25average groove width of widthwise groove Average groove depth ofcircumferential fine groove/ — 0.95 0.85 0.95 — 0.95 0.95 0.95 averagegroove depth of widthwise groove Average groove width of widthwise finegroove/ — 0.25 0.25 0.85 — 0.25 0.25 0.25 average groove width ofwidthwise groove Average groove depth of widthwise fine groove/ — 0.950.85 0.95 — 0.95 0.95 0.95 average groove depth of widthwise grooveAverage intersecting angle of widthwise fine groove ° 70 70 70 — 70 7070 with respect to equatorial line of tire Behavior of circumferentialfine groove in ground — closed not closed not closed — closed closedclosed contact Behavior of widthwise fine groove in ground contact —closed closed not closed — closed closed closed Height of raised bottomportion at side edge of tread mm — — — — 20 20 20 in circumferentialfine groove at tread center mm — — — — — 20 20 Decrease of height incorner part — — — — — absence absence presence

[0080] With respect to the wear resistance, each of the tires is mountedonto a front wheel of a construction vehicle and run at an approximatelyequal speed of 10 km/h for 1000 hours, and thereafter the tread isdivided into 8 parts in the widthwise direction of the tire andremaining groove degrees at these 8 positions are measured to calculatean average value Gw of rubber gauge of the tread worn through therunning. A value obtained by dividing the running time by the averagevalue Gw is evaluated as a value of wear resistance and represented byan index on the basis that the conventional tire 2 is control.

[0081] With respect to the resistance to crack growth, each of the testtires is mounted onto a driving shaft of a rear wheel of a constructionvehicle and run on an off road at an approximately equal speed of 10km/h for 1000 hours after cut failure reaching to the belt is formed inthe central zone of the tread to measure a length of crack growth. Avalue obtained by dividing the running time by the crack growth lengthis evaluated as a vale of resistance to crack growth and represented byan index on the basis that the conventional tire 2 is control.

[0082] With respect to the heat radiation, a hole of about 10 mm indiameter reaching to the belt layer is previously formed at a centralposition of a block located at each of 8 positions of the tread in thewidthwise direction of the tire, and then the tire is run on a drum of 5m in diameter at a speed of 10 km/h for 48 hours, and thereafter atemperature of each hole is measured through a thermocouple. The heatradiation is evaluated by a highest temperature among the measuredvalues.

[0083] Moreover, an atmosphere temperature is about 30° C., and theevaluation is carried out according to a standard that the temperatureof the conventional tire 2 is 0.

[0084] With respect to the traction performance, each of the test tiresis mounted onto a driving shaft of a rear wheel of a constructionvehicle and run on an off road at an equal speed of 10 km/h from a flatstate to a slope having an inclination degree of 8%, during which thetraction performance is evaluated by 5 stages according to a standardthat the conventional tire 2 is 4.

[0085] The evaluation results on each performance are shown in Table 2.Moreover, the larger the absolute value of each numeral, the better theresult. TABLE 2 Example Example Conventional Conventional ExampleExample Example tire 1 tire 2 tire 1 tire 2 tire 3 tire 4 tire 5 Wear160 135 100 100 165 168 170 resistance Resistance to 140 140 130 100 140140 140 crack growth Heat −1 −2 −4 0 −1 −1 −1 radiation Traction 5 5 5 45 5 5 performance

[0086] As seen from Table 2, the wear resistance and the resistance tocrack growth are considerably excellent in each of the example tires asscompared with each of the conventional tires.

[0087] As to the heat radiation, each of the example tires is poor ascompared with the conventional tire 1, but shows an excellent result ascompared with the conventional tire 2. This is considered due to thefact that since each of the example tires is large in the block volumeas compared with the conventional tire 1, the quantity of heatgeneration increases but the decrease of the groove volume is not solarge and hence the heat radiation is relatively good and the control ofthe temperature rise is advantageously realized.

[0088] As to the traction performance, each of the example tires shows aresult equal to those of the conventional tires.

EXAMPLE 2

[0089] A mold cost and an exchanging time of a split mold part aredetermined by changing a kind of molds and a boundary position betweenmold parts, while an uneven wear of a land portion in a product tire dueto the presence of spew rubbers produced at the boundary positionbetween the mold parts is measured at a time of running the tire at aspeed of 10 km/h over 5000 km to obtain results as shown in Table 3.

[0090] In this case, the tire size, use conditions and the like are thesame as in Example 1. TABLE 3 Conventional Comparative ComparativeExample Example 1 Example 1 Example 2 Example 3 Example 4 Example 5Example 6 Example 2 Kind of mold full mold full mold split mold splitmold full full full full full mold + mold + mold + mold + mold + splitmold split mold split mold split mold split mold Figure correspondingFIG. 9 (lug) to pattern Boundary position none none in center inwidthwise widthwise straight at in land in center block shoulder outeredge outer edge outer end portion of block block of groove of grooveportion of shoulder groove Mold cost *1 100 100 250 250 190 170 175 175175 Exchanging time of — — 100 100  85  85  85  85  85 split mold part*2 Take-out of possible impossible possible possible possible possiblepossible possible impossible vulcanized tire Uneven wear *3  94impossible  94  94 100 100  97  94 impossible (influenced by spew) toproduce to produce tire tire

[0091] As seen from Table 3, the exchanging of the split mold part isexcellent in all examples though there is a disadvantage in the moldcost, and also the smooth taking-out of the vulcanized tire can beattained. Particularly, the uneven wear due to the presence of spewrubbers can be effectively prevented in Examples 1, 2 and 6.

[0092] [Industrial Applicability]

[0093] According to the invention, the wear resistance and theresistance to crack growth in the off-the-road tire can be improvedwhile ensuring the excellent heat radiation.

1. An off-the-road tire having a gauge of a tread rubber within a rangeof 60-200 mm, in which a plurality of widthwise grooves extendingsubstantially in a widthwise direction of the tire are arranged in eachside region of the tread, and a pair of circumferential fine groovescontinuously extending straightforward or zigzag in a circumferentialdirection of the tire are arranged in a central zone of the tread, and aplurality of widthwise fine grooves extending substantially in thewidthwise direction of the tire and contacting their grooves walls witheach other in a ground contact area are arranged between thesecircumferential fine grooves.
 2. An off-the-road tire according to claim1, wherein a region corresponding to a bead portion and a sidewallportion and a tread shoulder portion of a product tire is vulcanizedthrough a mold part of a full mold form and a region corresponding to acentral portion of a tread located from the tread shoulder portiontoward a tread center is vulcanized through a mold part of a split moldform, respectively.
 3. An the off-the-road tire according to claim 2,wherein a boundary between the mold part of the full mold form and themold part of the split mold form is made in correspondence with a grooveedge of a straight or zigzag circumferential fine groove to be formed inthe tread and located toward a side edge of the tread, and thevulcanization is conducted in a mold of these mold parts located alongthe groove edge.
 4. An off-the-road tire according to claim 2, whereinthe vulcanization is carried out through a mold in which a boundarybetween the mold part of the full mold form and the mold part of thesplit mold form is straightforward positioned in correspondence with agroove edge of a zigzag circumferential fine groove to be formed in thetread and located nearest to a side edge of the tread.
 5. Anoff-the-road tire according to claim 2, wherein the vulcanization iscarried out through a mold in which a boundary between the mold part ofthe full mold form and the mold part of the split mold form ispositioned in correspondence with a land portion defined by thecircumferential fine groove to be formed in the tread toward the sideedge of the tread.
 6. An off-the-road tire according to claim 1, whereinthe widthwise groove has a groove width of 25-80 mm.
 7. An off-the-roadtire according to claim 1, wherein the widthwise groove has a groovedepth of 50-160 mm.
 8. An off-the-road tire according to claim 1,wherein a groove width of the circumferential fine groove is 15-50% ofthe groove width of the widthwise groove.
 9. An off-the-road tireaccording to claim 1, wherein a groove depth of the circumferential finegroove is 60-100% of the groove depth of the widthwise groove.
 10. Anoff-the-road tire according to claim 1, wherein the groove width of thewidthwise fine groove is 15-50% of the groove width of the widthwisegroove.
 11. An off-the-road tire according to claim 1, wherein thegroove depth of the widthwise fine groove is 60-100% of the groove depthof the widthwise groove.
 12. An off-the-road tire according to claim 1,wherein an average intersecting angle of the widthwise fine groove at anacute angle side with respect to the equatorial line of the tire is45-90°.
 13. An off-the-road tire according to claim 1, wherein a treadrubber in the central zone of the tread has a JIS A hardness of 55-80and a loss tangent tan δ (25° C.) of 0.05-0.35.
 14. An off-the-road tireaccording to claim 1, wherein the groove width of the circumferentialfine groove is rendered into a width contacting the groove walls witheach other in a ground contact face.
 15. An off-the-road tire accordingto claim 1, wherein each of blocks defined by the circumferential finegrooves and the widthwise fine grooves as a land portion is triangular,quadrilateral or hexagonal.
 16. An off-the-road tire according to claim1, wherein the vulcanization is carried out through a mold in which awidth of a projection for the formation of the circumferential finegroove is made smaller than a width of a projection for the formation ofthe widthwise fine groove.
 17. An off-the-road tire according to claim1, wherein a groove bottom of a straightforward extending portion in thezigzag extending circumferential fine groove is rendered into a raisedbottom at an end portion located toward a side edge of the tread.
 18. Anoff-the-road tire according to claim 1, wherein a groove bottom of astraightforward extending portion in the zigzag extendingcircumferential fine groove is rendered into a raised bottom at an endportion located toward the side of the tread center.
 19. An off-the-roadtire according to claim 1, wherein a surface height of a corner part inthe land portion adjacent to the side edge of the tread defined by thezigzag extending circumferential fine grooves is gradually decreasedtoward a top side thereof.